Transparent films displaying information are widely used throughout many different industries and for many applications. Typically, a positive image is formed by placing an ink or pigment onto a transparent plastic sheet. The image is then displayed by projection of transmitted light.
Phase change ink printing has been demonstrated to be a superior method of printing. Among the advantages offered by phase change ink printing is the ability to obtain a high optical density and large print areas without the necessity for removing large volumes of solvent after printing. The impact of phase change ink printing for transparencies has been impeded due to the lack of a suitable media. Transparent media designed for use with aqueous ink jet printers are often used but these exhibit insufficient adhesion between the phase change ink and the media.
Phase change inks are characterized by their ability to remain in a solid state at ambient to warm conditions yet melt to a liquid at the printing head operating temperatures. Exemplary printing apparatus are disclosed, for example, in U.S. Pat. No. 5,276,468. The physical thermomechanical properties of the solid glassy state, the solid rubbery plateau state and the liquid melt are all important in the design of the phase change inks and printers. Exemplary phase change inks are provided, for example, in U.S. Pat. No. 5,372,852.
Contrary to solvent ink systems the phase change ink resides predominantly on the surface of the media and does not appreciably diffuse into the matrix of the media or coating. This phenomenon has challenged skilled artisans to develop a media which has suitable adhesion with the phase change inks. Media presently known in the art generates too weak of an adhesive bond to withstand even moderate impact. The prints delaminate easily during normal use. This is particularly a problem when large areas are printed.
Three methods are known in the adhesive art which increase the strength of the adhesive bond. The first is to increase the polarity of the surface to create high surface energy. This increases adhesion to the ink by a thermodynamic driving force to lower the total interfacial energy. The second increases the dispersive forces between media and ink by coating a primer with properties intermediate between the base polymer sheet and the ink. Using the rule that "like dissolves like" better anchorage results. However, neither approach provides the high impact resistance needed to avoid delamination in the impacted area. The third approach commonly used to improve adhesion increases the surface area. However, this results in large increases in surface haze, making the media no longer transparent.
Printing phase change ink at high percent surface coverage can negate high surface haze by filling in a rough surface. Thus, it is possible to create clarity by overprinting clear phase change ink in low image density areas. Using this approach, the high surface area approach to increased phase change ink anchorage can be made to be essentially transparent after printing. But high surface area alone is not effective in increasing the impact resistance to acceptable levels, particularly if the porosity is not filled in by the ink, either by its being too narrow in radial dimension or too deep into the coating. What is required is a particular porosity with a large number of accessible pores with anchorage sites which provide lock points for the congealed phase change ink.
There is a need for a media which will take full advantage of the properties offered by phase change ink printing. Provided herein is a coated media which exhibits excellent adhesion to phase change ink, offers adequate clarity, and greatly improves durability of the printed image as measured by increased resistance to ink removal.
Ink removal can either be from scratching with a hard object, adhesive removal by contact of the ink with an adhesive-containing object such as an adhesive tape, or by impact and consequent delamination of the phase change ink from the media surface. The first type of failure is largely a function of rheology of the phase change ink and as such is not addressed in the present invention. However, to the extent that ink is imbedded into the media as described in the present invention, removal by gouging with a blunt or sharp, hard object can be improved. Ink removal by adhesive contact is affected by the adhesion to the ink surface which depends in turn on its surface energy and as such is not addressed in the present invention. However, to the extent that the result actually loads the ink/media interface, a porous surface with ink imbedded into these pores breaks up the continuous failure line resulting in improved retention of ink at peel-like frequencies.
U.S. Pat. No. 5,753,360, which is commonly assigned, defines a media which is suitable for ink jet printing media. The results are based on a tape test which is a relatively mild test for adhesion. A more strenuous test, based on physical impact, indicates that a far superior film can be obtained which is described herein.